A surge arrester of the above type is normally capable of passing surge currents without any arcing within the housing of the arrester. Under normal voltage conditions, the metal-oxide varistor elements have a high resistance that essentially blocks current flow therethrough; but should a voltage surge appear across the arrester, the varistor elements will respond to the rising voltage of the surge to switch to a low-resistance state that allows excess current to flow through the varistor elements, thereby limiting the voltage across the arrester and across any protected equipment that is connected in parallel with the arrester. Normally, this current through the arrester will be confined to the solid material of the varistor elements, and no arcing will occur within the arrester. Under unusual conditions, however, there is a possibility that one or more of the varistor elements will fail, and this will result in an electric arc being developed across the failed varistor element and, quickly thereafter, along the length of a varistor stack. Such an arc will rapidly generate extremely hot gases and relatively high pressures within the arrestor housing. To protect the arrester housing against being ruptured by such hot gases, it is conventional to provide for venting of the gases to the exterior of the arrester housing and, in effect, transfer the arc to an exterior location.
In conventional arrester designs, the exterior location to which the arc is transferred is along the length of the insulating housing of the arrester and between metal terminals located at opposite ends of the arrester housing. Arc transfer into such an exterior location requires venting of the arc-generated gases through relatively long and constricted passages that redirect the gases from each of the terminals to the opposite terminal. The use of such long, constricted passages for venting is disadvantageous because it lengthens the time required for arc transfer to the exterior and, moreover, requires a complex configuration of vent passages. The lengthened time for arc transfer to the exterior is disadvantageous because it increases the time that the arc remains within the housing interior and subjects the housing to rising and potentially-damaging pressures and temperatures. It is especially important to limit the time required for the arc to transfer to the exterior of the housing in applications involving very high fault currents, such as in applications where the arrester is relied upon to protect series capacitors in a series capacitor compensation scheme.